variable cathode resistor install question for vintage vox ac30

[jhanly]

Hi All,

I've installed a 200 ohm (12w) variable cathode resister (rheostat) in my ’68 AC30 to make biasing new output tubes easier.

I'm wanting to check if I’ve done things correctly before powering it up.

I’ve wired a 40R 10W resistor in series with rheostat. Specifically, the (shared) cathode connects to the 40R resistor which in turns connects to the wiper of the rheostat. From there I’ve wired one of the outside lugs of the rheostat to ground, completing the variable cathode resistor circuit. I’ve left the opposite (outer) rheostat lug free (open).

This obviously gives me a minimum of 40R between cathode and ground if the rheostat fails by shorting to ground - if that's ever a thing.

Having said that, I've just read that if/when a wiper from a rheostat comes off its track it's designed to engage the full load of the variable resistor (?).

Either way, my chosen wiring set up avoids the chance of a direct short between cathode and ground.

My choice of wiring is based on my assumption that the most catastrophic possible failure situation is if the shared cathode shorted directly to ground - allowing for excess levels of current to build in the output tubes. Conversely, if the cathode becomes open (completely disconnected) it prevents any and all potential for meaningful levels of current to flow in the tube - and while this latter failure will stop the amp from working it’s less likely to do any catastrophic damage to the output tubes and output transformer etc.

Or am I completely off track in that assumption?

Kind regards
James

[sluckey]

I would connect the unused lug of the rheostat to the wiper. Then if the wiper fails open the cathode will have still 40Ω + 200Ω to ground (as opposed to an open circuit if wired as you suggest). The amp will still continue to play at the coolest bias setting.

[pdf64]

So you’re intending on leaving the rheostat permanently in place, rather then removing it and fitting a suitable value resistor once you’ve found your preferred setting?

200 ohms seems a higher value here, leading to poor resolution. Consider that just 82R may be perceived as being the upper reasonable limit / too high a value, for the AC30 cathode resistor.

What V AC do you measure across the mains transformer 6.3V heater winding?

[jhanly]

Thanks Stuckey, will do that

Hi pdf64, yes the intention is to leave it in place - and see how that goes. I'm adjusting the rheostat to around ~10R so that with the 40R in series total cathode resistance is sitting around ~50R. I'm able to adjust in approx 1-2R increments (of sorts).

Cheers
James

[kagliostro]

I'll go for a bit more of 50R

I know, the AC30 is famous for 110% BIAS settings (the 47R version)

If I don't remember badly in one further models was 82R switchable with 50R

So I'll try something like 60R, your power tubes will thank you

Franco

[jhanly]

I hear you kagliostro, I've been sitting at ~75R for a while and after reading posts from the ~50R/100%+ brigade have been curious to warm things up a touch and see how it sounds. After all, 50R is the original value for the amp so I figure I'd give it a listen (albeit at maybe a slightly higher mains voltage than intended for the 50R value).

The rheostat makes this kind of tweak-and-see a simple procedure - including returning to a more conservative setting once I'm done experimenting.

60R sounds like a sensible centre and I expect I'll end up somewhere round there when my NOS set of el84's turn up.

Cheers

[kagliostro]

Just to see how VOX did it give a look to the power section of the AC30CC2

Franco

[dwinstonwood]

Just to see how VOX did it give a look to the power section of the AC30CC2

Franco

I agree with this. In fact, you can find two bias settings that you like the best using the rheostat, and then parallel a resistor of the correct value on a switch for the hot setting like Vox did.

[jhanly]

clever addition for sure